Briding amplifier



April 3, 1951 J. w. RIEKE 2,547,53fi

BRIDGING AMPLIFIER Filed Jan. 27, 1948 OUTPUT INPUT i Rl/ I INVENTOR By J. W R/EKE ATTORNEY Patented Apr. 3,

iTED S BRIDGING AMPLIFIER John W. Rieke, Astoria, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 27, 1948, Serial No. 4,489

6 Claims. (01. 179-171) This invention relates to electric wave amplifiers and particularly to amplifiers having balanced divided input circuits.

The object of the invention is an amplifier which will discriminate against longitudinal currents, and provide increased amplification for balanced signal currents.

A feature of the invention is an electric discharge device, having its anode-cathode path connected in the common branch of the divided input and divided output circuits of the amplifier, and its control electrode connected to one of the output circuits. This electric device should have a high impedance to alternatingvoltages applied to the anode, and a moderate direct resistance, and may conveniently be a pentode vacuum tube.

- The drawin schematically shows an electric wave amplifier embodying the invention.

A balanced transmission line may be con-' nected to the input terminals i, 2, causing signal currents to flow through capacitor Cl and resistors RI and Bit, and through capacitonCZ and resistors R2 and Bit. The voltages developed across grounded resistors Ri R2, are applied in push-pull, or differential relationship, through the small resistors R55, R! to the control grids of the vacuum tubes VI, V2. The vacuum tubes Vi, V2, preferably have high grid to anode transconductances and may conveniently be of the pentode type, though tetrodes and, in some cases, triode are satisfactory. A battery 5, or other convenient source of power, is connected through the small resistors RM, R5, to the screen grids of the vacuum tubes VI, V2; through the small inductor Ll, coupling resistor Hi2, and small resistor Rlt to the anode of vacuum tube Vi; and through the coupling resistor RH and small resistor Rt to the anode of vacuum tube V2. The coupling resistor Hi 2 is connected through capacitor Ct to the output terminal 3, while the output terminal is grounded. The suppressor grids of vacuum tubes VI, V2, if any, may conveniently be grounded. The cathode of vacuum tube Vl is connecmd through an adjustable resistor R5 to the cathode of vacuum tube V2. R! and RIB provide bias voltage for VI; R2 and Bill provide bias voltage for V2. The heaters of vacuum tubes VI, V2 are connected as usual, to some convenient source of power (not shown).

The vacuum tube V3 should have a high alternating impedance, and a moderate direct resistance and may conveniently beoi the pentode or tetrode type. The anode of vacuum tube V3 is connected to the cathode of vacuum tube V2; the

. good automatic self bias of tube V2.

2 I screen grid of vacuum tube V3 is connected through the small resistor R5 to the source 5;. the suppressor grid and cathode of vacuum tube V3 are connected through a biasing resistor R3 to ground; and the control gridof vacuum tube V3 is connected through the grid leak resistor Rd to ground. The heater of vacuum tube V3 is connected to some convenient source of power (not shown).

The anode of vacuum tube V2 is connected through the small resistor R9 and coupling capacitor C3 to the control rid of vacuum tube V3.

The anode current of tube V! flows from the grounded source 5 through inductor Li, resistors RiZ, R13, the anode-cathode path of tube Vi, and resistor R5; the anode current of tube V2 flows from the source '5 through resistors RH, R9, and the anode-cathode path of tube V2. The anode currents of tube VI V2, then flow through the anode-cathode path of tube V3 and resistor R3 to ground, back to the source 5. Tube V3 is biased by means of R3 so that the anode current of tube V3 is twice the desired cathode current of each of tubes V i and V2. Tube VI is biased by means of resistors RI and R15 to conduct a cathode current equal to one half the current required by the anode of tube V3. The remaining one half of the tube V3 anode current will be supplied by the cathode of tube V2 because the moderately high cathode to ground impedance provided for tube V2 by the cathode of tube V! and the anode of tube V3 results in Resistors R2 and RH] maintain the control grid to ground voltage of V2 at about one half the supply voltage 5, so that the anode to cathode voltage of all three tubes is substantially equal.

A modern pentode, such as the tubes Vi, V2, may have a transconductance of several thousand micromhos, thus, the impedance of the tube viewed into the cathode, that is, the impedance the tube presents to a voltage applied between the ground and cathode, is only a few hundred ohms. On the other hand, a modern pentode, such as tube V3, will have an impedance viewed into the anode, that is, the impedance which the tube presents to a variation in the voltage ap plied to the anode, which is of the order of one hundred thousand ohms, or more.

The small resistors R5, El, R8, R9, R53, Rid, RIB, prevent high frequency oscillations and, they have resistances of only somefiity ohms, may be ignored in discussing the operation of the amplifier.

The small inductor LI having an inductance of some 50 microhenries, or less, counteracts the effects of the parasitic capacitance-s in the amplifier and is selected to maintain more nearly constant the response-frequency characteristic of the amplifier at high frequencies, equalizing the response of the amplifier at the upper end of the characteristic. This inductor will have a resistance of a few ohms. The load resistors RI i, RIZ, may have resistances of the order of a thousand ohms.

When balanced-to-ground voltages are plied to terminals I and 2 of the amplifier they tend to produce anode circuit currents in tubes VI and V2 which are out of phase. These currents annul one another at the anode of tube V3 so that no current is required to flow through the anode circuit of V3 to ground. Thus tube V3 produces no degeneration to the amplification of balanced-to-ground input signal voltages,

For longitudinal noise currents, which tend to produce identical voltages, with respect to ground, on the control grids of the tubes Vi, V2, the resulting currents in tubes VI, V2, are in phase, and the currents from the cathodes of these tubes are compelled to flow through tube V3, source 5, thence through the respective load resistors to the anodes of tubes Vi V2. The high impedance presented by the tube V3 to these currents will produce a large amount of degeneration, thus, the amplifier discriminates between balanced signal currents and longitudinal noise currents, amplifying the currents out suppressing, the noise currents.

The alternating current from tube V2 produces a voltage drop in load resistor RlI which. is applied to the control grid of tube V3 by any suitable coupling means such as the capacitor C3 and leak resistor R4. This voltage will be in re.- verse phase to the signal voltage applied to the control grid of tube V2, and in phase with the i nal voltage applied to the control grid of tube VI. The resultant variation in the impedance of tube V3 will produce a powerful degenerative efifect on tube V2, so that the voltage ampl ficae tion of tube V? is reduced, and very little alterhating current will now how in tube V2. As the voltage applied to the signal grid of tube V3 is in phase with the voltage applied to the signal grid of tube Vi, the alternating current of tube VI tends to flow from the cathode of tube VI through resistor R5, tube V3, source 5, load induce tor Li and resistor R52 to the anode of tube V i. It may be shown that this connection of the signal grid of tube V3 to the anode circuit of tube f V2 produces a voltage amplification of the signal which is approximately double the voltage plification of the same circuit but without said connection.

As substantially all the amplified signal currents flow from the cathode of tube V I through resistor R the anode of tube V3, resistor R5 will produce a degenerative effect on the output of tube VI, thus, by adjustment of the resistance of resistor R5, the gain of tube Vi may be varied.

This amplifier may also be operated from an unbalanced line by connecting one of the terminals I or 2 to the line and connecting the other terminal to ground. In this case, signal voltages are applied to the control grid of one of the tubes VI or V2, and the control grid of the other tube is held constant at ground potential. From the detailed description given hereinabove, the operation of the amplifier under these conditions is obvious.

What is claimed is:

1. An amplifier including first and second electronic devices, each having at least an activated cathode, an anode and a control electrode, means connecting both said control electrodes in balanced relationship, a circuit connecting both said cathodes, a third electronic device having at least an activated cathode, an anode and a control electrode, means connecting the anode of the third device to the circuit connecting the cathodes of the other two devices, means coupling the cathode of the third device to the control electrodes of the other two devices, a source of anode power having a negative pole connected to the cathode of the third device, circuits connecting the anodes of the first and second devices in balanced relationship to the positive pole of the anode source, a load coupled to the anode circuit of the first device, and a connection from the anode of the second device to the control electrode of the third device.

2 The combination of claim 1 with a gain controlling adjustable resistor in the circuit connecting the cathode of the first device and the anode of the third device.

3. The combination of claim 1 in which all said devices are pentodes having screen grids connected to said source.

4. A bridging amplifier for connection to a balanced transmission line having two conductors transmitting signal currents and susceptible to undesired potential differences between both said conductors and ground, including first and second electronic devices each having at least an activated cathode, an anode and a control electrode, means coupling the control electrodes of devices respectively to said conductors, two impedance; connecting said control electrodes in balanced relationship to ground, a circuit connecting both said cathodes, a third electronic device having at least a grounded activated cathode, an anode, and a control electrode, means connecting the anode of the third device to the circuit connecting the cathodes of the other devices, a source of anode power having a grounded negative pole, impedances connecting the anodes ol; the first and second devices in balanced relationship to the positive pole of the anode source, a load coupled to the anode circuit of the first device, whereby the amplifier provides full amplification for signal currents but discriminates against the undesired potential differences, and a connection from the anode of the second device to the control electrode of the third device, whereby the amplification of the signal currents is increased.

5. The combination of claim 4 with a gain controlling adjustable resistor in the circuit connecting the cathode of the first device and the anode of the third device.

6. The combination of claim 1 in which all said devices are pentodes having screen grids connected to said source.

JOHN W. RIEKE.

REFERENCES QETED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,073,477 Green Mar. 9, 1937 2,232,212 Cary Feb. '18, 1941 2,233,961 Rockwell Mar. 4, 1941 2,424,893 Mansford July 29, 1947 

